Conventionally, with a view to improving fuel economy and power output of an internal combustion engine and reducing exhaust emissions therefrom, various kinds of valve control apparatuses have been proposed which variably control the opening and closing timing or the valve lift of intake valves and/or exhaust valves so as to attain intake and exhaust performance suitable for operating conditions of the engine. As one of such conventional valve control apparatuses, a type is known which changes the phase of an intake cam with respect to a camshaft to thereby continuously change the opening and closing timing of an intake cam (e.g. Japanese Laid-Open Patent Publication (Kokai) No. 7-301144). In this type of valve control apparatus, however, the intake valve opens over a fixed valve-opening time period, so that when the opening timing of the intake valve is determined, the closing timing thereof is automatically determined. This makes it impossible to attain the optimum valve-opening timing and the optimum valve-closing timing at the same time for all regions of the rotational speed of the engine and load on the same which change steplessly.
Further, as another type of conventional valve control apparatus (e.g. Japanese Laid-Open Patent Publication (Kokai) No. 62-12811) is known in which each of an intake cam and an exhaust cam is formed by a high-speed cam and a low-speed cam having respective predetermined cam profiles different from each other, and each cam is switched between the low-speed cam and the high-speed cam for use in low rotational speed and high rotational speed of the engine, respectively. In this type of valve control apparatus, however, the cam profile is changed between two stages, and hence the opening and closing timing and valve lift of the intake/exhaust valve are also merely changed between two stages. Therefore, this apparatus is also not capable of attaining the optimum valve-opening/closing timing and valve lift for all regions of the rotational speed and load.
Further, still another type of a valve control apparatus (e.g. Japanese Laid-Open Patent Publication (Kokai) No. 8-200025) is known which uses electromagnets to open and close intake valves and exhaust valves. In this valve control apparatus, two intake valves and two exhaust valves are provided for each cylinder, and these four intake and exhaust valves are actuated by respective electromagnetic valve actuating mechanisms (hereinafter, this valve control apparatus is referred to as “the fully-electromagnetic valve control apparatus”). Each electromagnetic valve actuating mechanism is comprised of a pair of electromagnets opposed to each other, an armature arranged between the electromagnets and connected to the intake/exhaust valve associated therewith, and two coil springs urging the armature. In this electromagnetic valve actuating mechanism, the energization of the two electromagnets is controlled to cause the armature to be attracted to one of the electromagnets in an alternating fashion to thereby open and close the intake/exhaust valve. Therefore, by controlling the timing of energization, the opening and closing timing of the intake/exhaust valve can be controlled as desired, whereby it is possible to realize the optimum opening and closing timing for all regions of the rotational speed and load and optimize fuel economy, power output, etc. It should be noted that when the two electromagnets are not energized, the armature is held in a neutral position by the balance of the urging forces of the two coil springs. In this fully-electromagnetic valve control apparatus, however, all the intake/exhaust valves are each actuated by the electromagnetic valve actuating mechanism, so that the electric power consumption becomes very large, which reduces the effects of the improved fuel economy. Further, the electromagnets and armature of the electromagnetic valve actuating mechanism are formed by magnetic substances, which results in an increase in weight and manufacturing cost of the apparatus.
As a solution to this problem, the present applicant has already proposed by Japanese Patent Application No. 20001-012300 a valve control apparatus (hereinafter referred to as “the first valve control apparatus”) which actuates only one of two intake valves provided for one cylinder by an electromagnetic valve actuating mechanism similar to that described above, and the other of the intake valves and exhaust valves by cam-type valve actuating mechanisms operating in synchronism with rotation of the engine. In this first valve control apparatus, the opening timing and the closing timing of the one of the intake valves are set as desired according to operating conditions of the engine by using the electromagnetic valve actuating mechanism, whereby the optimum opening and closing timing can be realized, and the improvement of the fuel economy and the enhancement of the power output are made compatible. Further, compared with the fully-electromagnetic valve control apparatus, the number of electromagnetic valve actuating mechanisms is reduced to one fourth, which contributes to the fuel economy through reduction of electric power consumption, and reduction of weight and manufacturing costs.
Another valve control apparatus proposed by the present applicant is also known which is disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 63-289208 (hereinafter referred to as “the second valve control apparatus”). The second valve control apparatus includes a cam-type valve actuating mechanism for opening and closing an intake valve via a rocker arm by using a cam provided on a camshaft, and an electromagnetic actuator for holding the intake valve in an open position. This electromagnetic actuator is comprised of one solenoid fixed to a cylinder head, an armature fixed to a valve stem of the intake valve, and an impact-absorbing spring arranged between the armature and a retainer, and according to operating conditions of the engine, energizes the solenoid when the intake valve has reached the open position to cause the attractive force to act on the armature, whereby the intake valve is held in the open position to control the closing timing of the intake valve.
However, although the first valve control apparatus alleviates the problem suffered by the fully-electromagnetic valve control apparatus, due to its use of the electromagnetic valve actuating mechanism for part thereof, there still remains room for improvement in the following points: This valve control apparatus necessitates one electromagnetic valve actuating mechanism for one cylinder, and hence two electromagnets for one cylinder. This results in increased electric power consumption, and decreases the advantageous effects of improvement of fuel economy thanks to the variable opening and closing timing of the intake valve, and compared with the ordinary cam-actuated type valve control apparatus, the weight and manufacturing costs are still large. Further, the maximum rotational speed of the engine available through the use of the electromagnetic valve actuating mechanisms is substantially determined by a spring constant of each coil spring. This makes it necessary to set the spring constant of the coil spring to a large value and accordingly electromagnets providing large attractive forces are also required to be employed, when the apparatus is applied to an internal combustion engine whose maximum rotational speed is high (e.g. about 9000 rpm). This results in an increased electric power consumption, and degrades fuel economy in low-to-medium rotational speed operating regions in which the engine is usually operated more frequently than in other regions, and makes it difficult to attain the improvement of fuel economy and the realization of higher rotational speed and higher power output in a compatible fashion.
Further, the second valve control apparatus is only required to arrange one electromagnet for one intake valve of each cylinder, and therefore has advantages over the first valve control apparatus in that it can further reduce the electric power consumption and improve the fuel economy. However, there remains room for improvement in the following points: In the second valve control apparatus, irrespective of whether the electromagnetic actuator is active or inactive, the weight of the armature and the spring force of the impact-absorbing spring always act on the intake valve. This increases the inertial mass of the intake valve in the inactive state of the electromagnetic actuator, which restricts the maximum engine rotational speed and the maximum power output. In this case, to increase the maximum engine rotational speed, it is necessary to increase the spring constant of the valve spring. This degrades fuel economy due to an increase in electric power consumption, and makes it impossible to attain the improvement of fuel economy and the realization of higher engine rotational speed and higher power output in a compatible fashion, or sufficiently reduce the weight and manufacturing costs. Further, in the case of this valve control apparatus, to mount the solenoid, the armature, the impact-absorbing spring therein, it is necessary to modify the designs of the cylinder head and intake valves, at inevitably very high expenses.
This invention has been made with a view to providing a solution to these problems, and an object thereof is to provide a valve control apparatus for an internal combustion engine that is capable of optimally setting the closing timing of an engine valve according to operating conditions of the engine while suppressing an increase in the inertial mass of the engine valve to the minimum, thereby attaining improvement of fuel economy, and realization of higher engine rotational speed and higher power output in a compatible fashion, and reducing costs and weight thereof.